In bed rest and space flight, bone losses occur primarily in the spine and lower body regions, with no changes in the upper extremities. This suggests that those regions with the greatest decrease in load compared to pre flight or pre bed rest lose the most bone. This loss of skeletal mass may prove hazardous to astronauts on flights of long duration, not only because hypercalciuria might lead to the formation of renal calculi during flight, but axial skeletal fractures may occur upon return to Earth's gravity. Several studies of resistive exercise indicate that bone density increases with training programs employing low repetition, high load weight lifting. Using ambulatory subjects to test a 12 week resistive exercise regimen, similar to the exercise we propose for this bed rest study, we found significant increases in lumbar, thoracic, and total spine bone mineral density. Our goal is to determine if an in-flight program of resistive exercise could counteract the musculoskeletal effects of disuse. Resistive exercise offers the advantage of directly preventing the decreased bone strain occurring in disuse and provides a regional countermeasure for the local bone loss. While the ambulatory results appear promising, validation of the exercise protocol must be conducted before implementing a similar resistive exercise protocol in space. We will determine the effectiveness of intense resistive exercise as a countermeasure to bone and muscle loss in a 17 week bed rest study involving 10 controls and 10 exercise subjects. We will employ a Horizontal Exercise Machine and a modified version of the exercise protocol employed in the above-mentioned ambulatory study. We will obtain pre bed rest measures of calcium balance, bone density, metabolic bone markers, lean body mass, muscle strength, and muscle volume, and compare these with measurements obtained during early, mid and late bed rest. These results will also be compared to results from our previous 17 week bed rest studies conducted without countermeasures.